The ability to perceive and respond to the changing environment of solar radiation is a fundamental feature of most living organisms. Flavin-type blue light receptors known as cryptochrome is a major group of photosensory receptors which mediate various blue light responses in higher plants. Despite the recent progress made in the identification of this type of photoreceptors, our understanding with respect to the molecular mechanisms by which a cryptochrome mediates plant response to blue light is very limited. The proposed research is based on our recent isolation and characterization of the Arabidopsis gene CRY2, the second member of the Arabidopsis cryptochrome gene family. CRY2 is unique not only because it appears to mediate different blue light responses from that regulated by CRY1, but also in that its own expression is regulated by blue light. The long term goal of our study is to understand how plants respond to blue light, and we will take the combined approaches of biochemistry, molecular biology, and genetics to achieve the specific aims of this proposal. We propose to characterize the biochemical properties of the CRY2 protein purified from either homologous- or heterologous- expression systems. We will also identify the mechanism(s) responsible for the blue light-induced regulation of CRY2 expression. The biological function of CRY2 will be investigated by isolation and characterization of Arabidopsis cry2 mutants impaired in the CRY2 structure gene using different genetic methods. Accomplishment of these specific aims will allow experimental dissection of the molecular mechanisms underlying the ability of CRY2 to mediate blue light responses, and it will make it possible to gain further understanding of a vital signal transduction process in eucaryotic organisms.